The flexible shaft according to the invention is suitable for transmitting a torsion force in many applications, particularly when used inside a casing tube. In particular, this case, discloses an air-conditioning cleaning device, which is known from, among others, Finnish patents 94221 and 108408 and from utility model 9567. In the device for cleaning air-conditioning ducts, a flexible shaft is used to rotate a cleaning brush. The brush device used in the solution comprises a motor-driven brush, which is supported on the wall of the duct with the aid of a carrier brush. The flexible operating arm comprises a casing tube and a braided steel cable fitted inside it. The flexible shaft is rigidly attached from the casing tube to the body of the drive motor. The drive shaft is rigidly joined to the shaft of the motor, so that torque is efficiently transmitted in the long, flexible shaft to the brush head. During cleaning, the brush head is operated in both directions.
A steel cable of this kind can be wound to either the right or the left. Its torque-transmitting capability is therefore not symmetrical. A second drawback with this known solution is that the steel cable is heavy, with the reel weighing as much as several tens of kilograms.